In Situ Validation of a Correction for Time-Lag and Bias Errors in Vaisala RS80-H Radiosonde Humidity Measurements

Radiosonde relative humidity (RH) measurements are fundamentally important to Atmospheric Radiation Measurement (ARM) Program goals because they are used in a wide variety of both operational and research applications, including initialization of numerical models and evaluation of model results, validation of remote-sensor water vapor retrievals, construction of water vapor climatologies and studies of climate trends, parameterization of cloud processes, and as input to radiative transfer calculations. A systematic dry bias in Vaisala radiosonde humidity measurements has been noted in comparison to Raman lidar measurements (Ferrare et al. 1995) and satellite water vapor retrievals (Soden and Lanzante 1996), and in underpredicting clouds and precipitation in a numerical weather prediction model (Lorenc et al. 1996). Direct observations of bias error in Vaisala radiosonde humidity measurements have also been reported: dual-radiosonde launches at the Oklahoma ARM site showed offsets between radiosondes from different calibration batches that measured the same air (Lesht 1998); unrealistically dry tropical boundary layers were frequently observed during the Tropical Ocean Global Atmosphere-Coupled Ocean Atmosphere Response (TOGA-COARE) experiment (Zipser and Johnson 1998); and humidities far below ice-saturation were measured in cirrus clouds when simultaneous measurements of detailed ice crystal characteristics indicated that the crystals were pristine and therefore actively growing in an ice-supersaturated environment (Miloshevich et al. 2001a).